The water with a lot of dissolved minerals and salts is much denser than fresh water. And with a higher density, that means that less water needs to be displaced in order to float. This difference in density can be the difference between barely floating on your back with your nose barely out of the water versus floating comfortably on your back with your entire face out of the water. For instance, fresh water at 25C has a density of 997 kg/m^3, whereas the water in the Great Salt Lake, depending upon the salinity ranges from 1035 kg/m^3 to 1200 kg/m^3. Or in simpler terms it can be about 4% to 20% denser than fresh water and you'll float 4% to 20% higher in the salt water.
